During the course of recent Third Generation Partnership Project (“3GPP”) Long Term Evolution (“LTE”) standards-setting discussions, members have made several proposals for improving Multi-User Multiple-Input Multiple-Output (“MU-MIMO”) scheduling efficiency. These proposals can be categorized as follows.
Option 1: The evolved Node B (“eNB”) configures multiple Channel State Information (“CSI”) processes with suitable rank restrictions.
Option 1-1: The eNB configures two CSI processes. The first process has a codebook subset restriction that restricts the User Equipment (“UE”) to feed back the best rank 1 Precoding Matrix Indicator (“PMI”) and Channel Quality Indicator (“CQI”). The second CSI process has a codebook subset restriction that restricts the UE to feedback the best rank 2 PMI and CQI.
Option 1-2: The eNB targets the first CSI process towards Single-User MU-MIMO (no codebook subset restriction). The eNB targets the second CSI process towards MU-MIMO (rank 1 codebook subset restriction).
Option 2: The UE selects the best desired PMI and the best companion PMI corresponding to a co-scheduled user.
Option 2-1: The UE selects the desired PMI and the co-scheduled user PMI jointly such that the desired Signal to Interference plus Noise Ratio (“SINR”) or sum-rate is maximized. This option has a high implementation complexity.
Option 2-2: The UE adopts a suboptimal approach in which it selects the best desired PMI and then selects a companion PMI that leads to the smallest co-scheduled user interference. This option has a lower implementation complexity.
Option 3: The UE determines a Multi-User Channel Quality Indicator (“MU-CQI”) corresponding to K>1 (e.g., K=5) companion PMIs. The UE must evaluate a MU-CQI corresponding to each of the multiple companion PMI hypotheses. The UE draws the companion PMI hypotheses from a codebook subset that is either signaled or specified, so that the elements of the subset have a column space that is (quasi-) orthogonal to the column space of the desired precoder.